PROJECT SUMMARY Autosomal recessive mutations in the TBCK gene cause intellectual disability of variable severity. I have further characterized the neurologic phenotype of Puerto Rican children with a homozygous null mutation (p.R126X) in TBCK, which we designated the Boricua mutation. The biological mechanism underlying the genotype-phenotype correlations remain unclear. On one extreme, patients with the severe Boricua mutation develop progressive brain, cerebellar and motor neuron atrophy, coarse facial features and epilepsy. We named this severe syndrome TBCK-encephaloneuronopathy (TBCKE). On the other hand, other patients with biallelic TBCK mutations have clinical diagnosis of autism and/or intellectual disability, without evidence of neurodegeneration. The function of TBCK protein is unknown, but previous studies have shown absence of TBCK leads to downregulation of mTORC1 signaling. The mTORC1 pathway regulates autophagy, including the targeted degradation of mitochondria (mitophagy). I recently reported increased autophagic flux and impaired glycoprotein degradation in TBCKE patients? fibroblasts, which was rescued by activating mTORC1 signaling with L-leucine. Our fibroblasts studies suggests that TBCKE patients have mitochondrial dysfunction and mitochondrial DNA (mtDNA) depletion. Furthermore, the degree of mtDNA depletion predicts the neurologic severity of TBCK disease. Therefore, I hypothesize that loss of function of TBCK in human neurons leads to mtDNA depletion and mitochondrial dysfunction due to excessive autophagic clearance of mitochondria. To test this hypothesis in more disease relevant models, I propose to generate TBCK-null induced pluripotent stem cell (iPSC) derived neurons (iNeu) and tbck-/- zebrafish. The goal of this proposal is to address whether loss of function of TBCK affects mitochondrial function in human neurons, and to determine whether mtDNA depletion modulates the severity of neurodegeneration in TBCK disease. Using novel disease models and unique tools to assay mitochondrial function, I propose to address the following questions: (Aim 1) Do human neurons lacking TBCK protein have excessive mitophagy? (Aim2) What is the function of TBCK? Can we define its protein-protein interactions in neurons? (Aim 3) Can TBCK- null zebrafish model the variable severity of TBCK disease? Can mtDNA depletion modulate the severity of the phenotype in vivo? The experiments outlined in this proposal will determine the role of mitochondria in TBCKE and whether mtDNA depletion is sufficient to drive the neurodegenerative phenotype or merely an epiphenomenon. This work will also provide training in novel techniques for assaying neuronal mitochondrial function in disease models in situ. I will also learn to develop and characterize zebrafish models of neurodevelopmental disease. Support from this K02 award will be instrumental in growing my independent research program as a physician scientist in a superb institutional environment towards my long-term goal of studying the role of mitochondrial dysfunction in pediatric neurodegenerative disorders.